The present invention relates to a MgAl.sub.2 O.sub.4 spinel, and a process for the synthesis thereof, and a method of using the MgAl.sub.2 O.sub.4 spinel.
British Patent No. 1,296,049 discloses a process for synthesizing a stoichiometric magnesium/aluminum spinel. The synthesis of the spinel starts from a starting mixture which contains magnesium compounds and aluminum compounds. This starting mixture is calcined with oxygen to form magnesium and aluminum oxides. In the precursor mixture of the oxides which forms, the ratio of the magnesium used to the aluminum used is between 1.001:2 and 1.217:2, i.e. is superstoichiometric. From this precursor mixture composed of, inter alia, the oxides, the Mg/Al spinel of stoichiometric composition is synthesized by heating the precursor mixture, with periclase (MgO) also being formed in addition to the spinel, as a result of the excess of magnesium oxide.
For the formation of periclase, see DE-A 2,541,141, p. 15, Example 3. The spinel form is milled, pressed into a solid body of a desired shape and sintered. The sintered solid body is then used, for example, as a crucible for producing .beta."-aluminate solid electrolytes of sodium/sulphur batteries. In addition to the unwanted periclase, which causes interference, a further disadvantage of this known process is that the Mg/Al spinel synthesized thereby is sensitive to sodium oxide vapors which are formed, for example, in the production of these solid electrolytes for sodium/sulphur batteries.
U.S. Pat. No. 4,273,587 discloses a process for preparing an Mg/Al spinel, although this spinel has a substoichiometric proportion of magnesium. To prepare this spinel, an MgO/Al.sub.2 O.sub.3 starting mixture is heated to from 1150.degree. C. to 1300.degree. C., with the MgO being present in a substoichiometric amount. The spinel thus obtained from this starting mixture by calcination is milled and sintered to give a solid body of the desired shape. To obtain a high mutual mobility of the powder particles and to ensure good sintering of the grain boundaries of the spinel powder, between 0.001 and 0.1% by weight of LiF is added in the sintering of the pulverulent spinel. The stability of this spinel material to heated alkali metal oxides, particularly to sodium oxide, is low, as is also the case for the above-mentioned MgAl.sub.2 O.sub.4 spinel of stoichiometric composition.
U.S. Pat. No. 4,542,112 likewise discloses a process for preparing an Mg/Al spinel having a substoichiometric proportion of magnesium, with the starting materials used in this process being alkoxides containing Mg and Al. The ratio of the alkoxides in the starting mixture, to which LiF is added prior to hydrolysis to MgO and Al.sub.2 O.sub.3, has an excess of aluminum. This ratio, which is substoichiometric with respect to magnesium, is necessary to avoid periclase, since otherwise. That is, with a superstoichiometric proportion of magnesium oxide, an interfering periclase (MgO) phase is formed in addition to the desired spinel phase in the subsequent spinel preparation.
It is, therefore, an object of the present invention to provide an MgAl.sub.2 O.sub.4 spinel which is as free as possible of periclase and which is stable to hot alkali metal oxide vapors. In addition, it is an object of the present invention to provide an improved process for preparing such a MgAl.sub.2 O.sub.4 spinel, and a novel use of the spinel.
These objects have been achieved according to the present invention by providing a spinel having a superstoichiometric proportion of magnesium and by a process in which from one to five percent by weight of a reactive salt containing fluorine and boron, based on the metal oxides of the Mg.sub.(1+x) Al.sub.2 O.sub.4 spinel, are added to the precursor mixture before formation of spinel, mixing the precursor mixture for from 0.5 to 3 hour and, for the synthesis of the Mg.sub.(1+x) Al.sub.2 O.sub.4 spinel which is superstoichiometric with respect to magnesium, heating the mixed precursor mixture between 0.5 and 4 hours at a temperature between 1150.degree. and 1400.degree. C. As a result of the early addition of an additive, particularly a reactive salt such as LiF or NaF, to the aluminum oxide-containing starting components having a mainly face-centered cubic crystal lattice, on heating, positively charged atoms of the additive (e.g., Na.sup.+ or Li.sup.+) occupy some sites of the hydrogen atom (H.sup.+) leaving the crystal structure. The occupation of some hydrogen lattice sites by the cations stabilizes the highly symmetrical face-centered cubic host crystal lattice of the heat-treated powder formed from the aluminum oxide-containing starting materials, even at temperatures above 1200.degree. C. A temperature of 1200.degree. C. is of interest because, on the one hand, the host crystal lattice is usually destroyed above this temperature by the driving-off of the hydrogen and, on the other hand, it is only above about this temperature that significant Mg/Al spinel synthesis occurs.
On addition of boron ions (B.sup.+), the boron replaces the aluminum (Al.sup.3+), producing a charge asymmetry or a vacancy in the lattice, which facilitates the incorporation of the divalent magnesium ion (Mg.sup.2+). However, the maintenance of a host crystal lattice similar to the face-centered cubic Mg/Al spinel is of importance for the superstoichiometric incorporation of the magnesium during the preparation of the spinel, since this incorporation is made easier or made possible in the first place if the starting materials have a host crystal lattice (e.g., face-centered and cubic) which is largely similar to that of the Mg/Al spinel to be prepared (body-centered and cubic, or hexagonal). In addition, the maintenance of the host crystal lattice, despite the superstoichiometric proportion of magnesium oxide in the precursor mixture and thus contrary to conventional thinking, also surprisingly suppresses the formation of the periclase phase in the Mg/Al synthesis.
Increasing the temperature-stability of the host crystal lattice is furthermore also favorable since, as can be seen from a phase diagram of an Mg/Al spinel, this spinel is formed only above 1150.degree. C. In addition to the partial substitution of the hydrogen lattice sites by the cations, there is also a partial substitution of the lattice sites of oxygen by the anions, for example by the univalent fluoride. This creates vacancies which make the incorporation of the magnesium easier. The incorporation and the raising of the superstoichiometric proportion of the magnesium in the spinel is thus a direct consequence of the timely addition of the fluorine- or boron-containing additive which serves to stabilize the host crystal structure and to raise the concentration of the oxygen vacancies in the heat-treated starting materials.
The high proportion of magnesium is of particular interest in spinels which are used for crucibles for producing .beta."-alumina ceramic electrolytes sodium/sulphur high-temperature batteries, since the heated sodium oxide usually attacks the spinel of the crucible. Here the superstoichiometric proportion of magnesium according to the invention has the effect of the magnesium of the spinel protecting against alkali metal oxides, in particular the heated sodium oxide. Furthermore, the spinel and any articles which may be sintered therefrom is convenient to produce.